This invention relates generally to pneumatic impact hammers of the type having an impact piston and a fluid delivery tube received within a coaxial bore in the piston for supplying pneumatic pressure fluid to one or both ends of the piston operating chamber for reciprocation of the piston.
U.S. Pat. No. 5,205,363 discloses a pneumatic impact drill having a porting system for automatically supplying pneumatic pressure fluid from a fluid delivery tube received within a coaxial bore in the impact piston to the opposite ends of the piston operating cylinder as the piston reciprocates. The porting system comprises an annular set of equiangularly spaced outlet ports in the fluid delivery tube and two sets of radially extending bores in the piston having first and second, axially spaced, annular sets of equiangularly spaced inlet ports which cooperate with the outlet ports in the delivery tube. The number of inlet ports in the piston and the number of outlet ports in the supply tube are selected to provide substantial fluid communication therebetween at all relative angular positions of the piston and fluid delivery tube.
Such invention provides a new and improved porting system. However, this porting system uses a single fluid delivery tube port to complete alternative flow paths for the two axially spaced piston ports and requires axial piston movement to initiate transfer from the first flow path to the second flow path. Consequently, pressure conditions that impede the movement of the piston can exist while the fluid delivery tube port is positioned intermediate the two piston ports.